about 35 yrs old). It seems like the old subfloor rests on this piece of concrete and it causes that part of subfloor to be 5/8 higher than new subfloor. The difference in height seems less noticable as you cross the room. Between old tongue and groove subfloor and newer 3/4 " plywood ,it is piecemealed in what looks like an attempt to level. I am thinking if the concrete is ground down to make original subfloor level with new ,it would be ok but I am sure I am over simplyfying here. Could it have been level and then sunk? There is basement under original but nothing under new. Any insight here would be helpful.